This invention relates in general to variable amplification or attenuation and, more particularly, to a method and apparatus for effecting programmable gain amplification.
There are circuits in which it is useful to have an amplifier or attenuator with a variable gain, such as a programmable gain amplifier. One existing application for such a device is to provide broadband Internet access in an upstream cable modem driver for a cable modem system. One possible approach is to use a high speed multiplexer, but this introduces switches into the signal path, which degrades performance.
Another existing approach uses a Gilbert cell to implement an attenuator. A problem with this approach is that a class A quiescent current does not decrease as the output signal is attenuated. This leaves the shot noise constant as the signal level at the output falls due to attenuation, thus degrading the signal-to-noise ratio as attenuation increases. In addition, if the cell is used as an output, it has an output voltage compliance which is not entirely satisfactory. Although it is possible to use current mirrors to improve the voltage compliance, this puts more active electronics in the signal path, which in turn causes a further degradation in distortion, noise, linearity, and bandwidth.
Although such pre-existing approaches have been generally adequate for their intended purposes, they have not been satisfactory in all respects. Power consumption, noise and distortion are higher than ideally desired, as is the glitch energy which occurs at the output when the device is enabled or disabled. Further, the bandwidth is narrower than desired for an application such as an upstream cable modem driver.
From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for obtaining programmable gain amplification, with some or all of the advantages of low power consumption, low noise, low glitch energy, low distortion, and high bandwidth.
According to a first form of the present invention, a method and apparatus are provided to address this need, and relate to a programmable gain amplifier which has a plurality of transconductor sections that each have an input node, an output node and an enable node. The method and apparatus involve: applying to the enable node of each transconductor section a respective one of a plurality of enable signals that can each be selectively set to have one of first and second states; applying a common input signal to the input nodes of each of the transconductor sections; causing each transconductor section to inhibit current flow at the output node thereof when the corresponding enable signal is in the first state; causing each transconductor section to produce at the output node thereof an output current when the corresponding enable signal is in the second state, the output current being proportional to a voltage of the input signal according to a gain, with the gains of the transconductor sections being different; and forming an output signal by summing the output currents from the output nodes of the transconductor sections.
According to a different form of the present invention, a method and apparatus are provided to meet the need, and relate to a transconductor circuit having an input node, an output node, and circuitry coupled between the input and output nodes. The method and apparatus involve: operating the circuitry so that it responds to an input voltage at the input node by generating an output current at the output node; and providing within the circuitry a transistor which has first, second and third terminals, the second terminal being a control terminal, the second and third terminals each being coupled to other components of the circuitry, and the first terminal serving as the output node and being free of connections within the circuitry.